1. Field of the Invention
This invention relates to a vibration sensor for sensing vibrations of, for example, an internal combustion engine to detect knocking and, more particularly, to a case for accommodating such a vibration sensor.
2. Description of the Related Art
FIG. 3 shows a partially sectional side view of a vibration sensor of this kind. As illustrated in FIG. 3, a base case 1 which is one of two outer members of the vibration detector and which is formed of steel has a threaded portion 1a and a bottom surface 1c for attachment to an internal combustion engine (not shown). A vibration member 2 is formed of a metallic plate and has a vibration surface 2a as well as a fixing peripheral portion 2b. A disk-like piezoelectric element 3 for converting vibration into an electrical signal is bonded to a side of the metallic plate opposite to the vibration surface 2a and is positioned coaxially with threaded portion 1a. A resin cover 5 forms the second outer member and is thermo-formed with an intermediate portion of a steel terminal 6 embedded therein. The resin cover 5 has a fixing peripheral portion 5a. The terminal 6 is insulated from a reference electrode by the resin cover 5. The terminal 6 is connected to an upper electrode of the piezoelectric element 3 by a lead wire 4. The vibration plate 2, a disk spring 7 and the cover 5 are successively inserted in the base case 1 and are fixed by a caulked portion 1b. A fixed end of the vibrating portion of the vibration plate 2 is thereby created. Spaces 11a and 11b are formed in the base case 1 and the resin cover 5, respectively. A potting recess 12 is defined by the caulked portion 1b and the resin cover 5.
The conventional vibration sensor thus constructed is fixed on an internal combustion engine by means of the thread 1a of the case 1, with the seat surface 1c abutting on the engine. Vibration generated according to the operating condition of the internal combustion engine propagates to the vibration detector through the bottom surface 1c. The vibration of the case 1 is then propagated to the vibration plate 2 and to the piezoelectric element 3. The piezoelectric element 3 receives a stress due to the vibration, generates a detection signal proportional to the stress, and outputs a terminal 6 detection signal with respect to the electrode (not shown) on the vibration plate bonding side. Since the case 1 is a metallic member, it has the same potential as the vibration plate 2. The vibration sensor has a natural frequency such that the output is maximized when it resonates with a component of knocking vibration of the internal combustion engine. This natural frequency is determined by the properties of the vibration plates 2 as well as the piezoelectric elements and the stability condition of the caulked portion 1b.
The space 11a provided on the base 1 side of the vibration plate 2 and space 11b provided on the resin cover 5 side are closed by the base case 1 and the resin cover 5, respectively, which together form a complete outer casing of the vibration sensor. Airtightness of the connection (at the caulked portion 1b) between the base case 1 and the resin cover 5 with respect to the thermal expansions of air in the spaces 11a and 11b is important in terms of reliability.
To ensure desired airtightness, a bonding agent or a potting agent is applied to this connection so as to fill the recess 12 formed by the caulked portion 1b of the base case 1 and the peripheral portion 5a of the resin case 5. However, the caulked portion 1b has an L-like caulked shape as illustrated, and the capacity of the recess 12 is small as determined by a depth H1. The reliability of this airtight connection is proportional to the capacity of the potting agent having a sealing effect. It is therefore desirable to further increase the capacity of the recess 12.
Thus, in the conventional vibration sensor constructed as described above, the capacity of the recess 12 is small and the airtight performance is limited according to this capacity. It is necessary to increase the capacity and to improve the reliability.
For example, a vibration detector attached to a motor vehicle typically undergoes a change of about 0.3 atm in the pressure of the closed spaces, such changes in pressure severely affecting the detector's durability.
A vibration plate, such as the plate 2A shown in FIG. 4, may be used for the vibration sensor of FIG. 3. This vibration plate 2A has a plurality of cutouts 2c circumferentially spaced around an outer periphery of the disk in order to improve its characteristics with respect to temperature fluctuations. The outer circumferential ends of the cutouts 2c extend to the peripheral portion 5a of the cover 5 such that the spaces 11a and 11b on the opposite sides of the vibration plates 2A communicate with each other. The airtightness of the portion in which the steel terminal 6 is embedded is reduced by the thermal expansion of air in the spaces 11a and 11b. There is therefore the problem of a reduction in reliability of vibration detection.